Trona deposits in Sweetwater and adjacent counties in Wyoming are found at depths of about 1500 feet underground and consist of a main trona bed varying from 8 to 10 feet in thickness. The trona consists mainly of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2 H.sub.2 O) containing about 3 to 15% or, more generally, 5 to 10% of insoluble impurities, mainly shale.
A typical natural trona composition is given below:
______________________________________ Na.sub.2 CO.sub.3 41.8% NaHCO.sub.3 33.1% H.sub.2 O 14.1% 89.0% Insolubles: Dolomite CaCO.sub.3 . MgCO.sub.3 5.5% Quartz SiO.sub.2 1.1% Feldspar (K,Na) 20 . xAl.sub.2 O.sub.3 . ySiO.sub.2 . zH.sub.2 O 3.3% Clay 2K.sub.2 0.3MgO . 8Fe.sub.2 O.sub.3 . 24SiO.sub.2 . 12H.sub.2 0.6% Shortite Na.sub.2 CO.sub.3 . 2CaCO.sub.3 0.1% Organic Matter as Elemental Carbon 0.2% Other (by Difference) 0.2% ______________________________________
Additionally, the trona usually also contains small amounts of soluble chlorides and sulfates. Two methods for obtaining soda ash from trona are in commercial use. The first involves calcination of crude trona at elevated temperature to yield crude sodium carbonate: EQU 2(Na.sub.2 CO.sub.3.NaHCO.sub. 3 .2H.sub.2 O).fwdarw. 3Na.sub.2 CO.sub.3 + 5H.sub.2 O+ CO.sub.2.
the crude sodium carbonate thus obtained is not of sufficient purity for commercial use. Hence, it is usual practice to purify it by crystallization. The second method for obtaining commercial grade soda ash from trona involves dissolving the trona directly, to separate purified sesquicarbonate crystals from the solution, and to calcine these crystals to transform them into commercial grade soda ash.
Calcination of trona effects moisture removal, reduction of elimination of organic matter as well as transformation of the sodium sesquicarbonate into sodium carbonate. Effectiveness of reduction of organic matter increases with increasing calcination temperature. Unfortunately, however, calcination at elevated temperature also causes sodium carbonate to react with silica contained in the trona to form soluble silicates, and formation of such soluble silicates also increase with increasing calcination temperature.
Soluble silicates are a major impurity in mother liquors from which sodium carbonate precursor crystals are obtained by evaporative crystallization. Unless steps are taken to reduce soluble silicate content of such mother liquors, soluble silicates become a significant impurity in the soda ash product. In the past, levels of soluble silicates in such mother liquors as well as levels of other contaminants, such as NaCl and Na.sub.2 SO.sub.4, have been controlled or maintained by purging part of the mother liquor. This, of course, entailed loss of valuable soda values.
It has already been proposed to reduce contamination by soluble silicates of aqueous sodium carbonate solutions obtained by dissolving calcined trona in aqueous media by digesting the solutions at elevated temperature above about 160.degree. F. for time sufficient to substantially reduce the soluble silicates contained therein. Apparently, there is some interaction between the insoluble impurities and the soluble silicates in such solutions, rendering insoluble or absorbing the soluble silicates to reduce their concentration in the solution on digestion. In our copending U.S. application Ser. No. 587,585 filed of even date herewith, there is described an improvement in the process of making sodium carbonate from trona by the method involving crushing the trona, calcining it and dissolving it in an aqueous medium for purification by crystallization, which improvement results in reduction of silicate contamination of the crystallization liquor and which improvement involves (a) segregating the calcined trona into a coarse and a fine particle size fraction; (b) separately dissolving the coarse and the fine particle size fractions to obtain aqueous solution of sodium carbonate, sodium silicates and insoluble impurities; followed by (c) digesting the solution of the fine particle size fraction at elevated temperature to insolubilize soluble silicates, and separating the digested solution from insoluble impurities.